1. Field of the Invention
The invention relates to an integrated services digital network (ISDN) and a station for an integrated services digital network, the network comprising a feeding unit for supplying a message, and including a station control circuit.
2. Discussion of the Related Art
In the integrated services digital network the digital messages (for example, speech, text, picture) are transported through a 64 kbit/s channel (B-channel). Such a B-channel is continuously switched from one terminal to another. For a subscriber line (basic access) there are two B-channels and one D-channel (16 kbit/s) available for signalling information. The signalling procedures are carried out over the D-channel according to a uniform standard. Signalling information is then inserted into layer-3 packets. Such a layer-3 packet has a header field with a protocol discriminator, a reference management and a message type as well as the actual payload in the payload field. For example, a SET-UP instruction for setting up a connection is transmitted in the layer-3 packet.
Furthermore, there is the possibility of connecting a subscriber unit, for example a private branch exchange, with a plurality of connected terminals, to a primary rate access. The primary rate access presents thirty user information channels with 64 kbit/s, one signalling channel and one synchronization channel. The coupling between a user information channel of the primary rate access and a terminal of the subscriber unit forms an exchange contained in the subscriber unit.
For the purpose of providing information, for example, about telephone numbers, there is also an inquiry facility available in the integrated services digital network. This inquiry facility has an exchange, a plurality of terminals connected to the exchange and an information-processing circuit which supplies, for example, telephone numbers to the exchange.
A station is understood to mean, for example, a single terminal, a subscriber unit and an inquiry facility.
An ISDN terminal comprises a telephone receiver and a microphone comprising each a bandpass filter and an analog-to-digital converter, or digital-to-analog converter respectively, for connection to a B-channel. The other B-channel may be used for data transmission, for example, via a serial or parallel data interface. Furthermore, a terminal control circuit (for example, microprocessor) is available which feeds signalling information to the D-channel, sends the signals to a display device and receives the signals from a feeding unit. A multiplexer/demultiplexer separates or combines respectively, the two B-channels and the D-channel.
For transmitting messages such as, for example, a telephone number, the subscriber can call upon the ISDN user-to-user signalling service during a call, which costs money (cf. for example: "Grundlagen der Vermitlungstechnik" by Gerd Siegmund, Decker Verlag, Heidelberg, 1991, pp. 213 to 232). To transmit information a message is then inserted into a layer-3 packet.